Silver tarnishing takes place through a variety of mechanisms. In general, this leads to a disfiguring layer on the surface of the silver that is visually unacceptable. The main product of silver tarnishing is silver sulfide caused by the presence of sulfides, such as hydrogen sulfide, present in the atmosphere. The reaction mechanisms are 8Ag+4HS−4Ag2S+2H2+4e− and O2+2H2O+4e−4OH−. The first reaction is believed to occur in a thin film of water on the silver surface. In dry air, tarnishing does not take place. In the second reaction, oxygen acts as a cathodic species and consumes electrons as indicated in the equation. Higher concentrations of hydrogen sulfide increase tarnishing. Although the rate of tarnishing gradually declines with increased tarnish layer thickness, the reaction proceeds even on a heavily tarnished surface, since, owing to its coarse structure, the silver sulfide does not form a protective layer against surface corrosion. When relative humidity (rh) is between 5 and 50%, the amount of absorbed water on the surface is approximately constant and the reaction rate is steady. However, between 70 and 80% rh, surface moisture increases and accelerates the reaction rapidly.
Various attempts have been made over the years to address the problem of silver tarnishing in the jewelry and electronics industries. U.S. Pat. No. 1,934,730 discloses a method of preventing silver tarnishing by forming an alloy of 55.5% silver, 36% indium and 8.5% gold. Gold was added as one of the alloying metals because silver and indium alloys typically form an undesirable bluish tint to the alloy. However, with the price of gold as high as it is the industry is discouraged from using such an alloy. A number of conventional methods coat silver with a layer of chromium from hexavalent chromium electrolytes. However, such methods have been strictly restricted due to the hazardous and toxic nature of chromium to workers in the industry as well as to the environment. Organic anti-tarnish films, such as self-assembled monolayers of organo-thiolated molecules such as n-alkanethiol and thioaromatic molecules, have been used as a substitute in some cases but they typically lack thermal stability and the lubrication properties of the organic films further limit their uses such as where application temperature is relatively high or lubrication effect is undesired. For example, in radio frequency (RF) connector applications the lubrication effect may cause undesired vibrations between two mating parts.
U.S. 2011/0151276 discloses a method of inhibiting silver tarnishing by depositing a silver and indium alloy of 90-99 wt % silver and 1-10 wt % indium by physical or chemical vapor deposition methods. An oxide layer of SiO2, TiO2 or Al2O3 may be coated on the silver and indium alloy to further improve tarnish inhibition. A disadvantage of depositing metals by physical and chemical vapor deposition is that it is difficult to deposit metals on parts having irregular geometry, such as internal surfaces of tubes. In addition, depositing metals by physical or chemical vapor deposition is more costly than by plating.
TW 201103177 discloses a process for inhibiting silver tarnishing by plating an indium layer on silver followed by heating at temperatures of 150° C. to 600° C. to form a silver and indium alloy.
Although there are methods of inhibiting silver tarnishing, there is still a need for improved methods of inhibiting silver tarnishing.